The units of selection and measures of fitness

This paper presents a unified account of the properties of the measures, Malthusian parameter and entropy in predicting evolutionary change in populations of macromolecules, cells and individuals. The Malthusian parameter describes the intrinsic rate of increase of the population. The entropy describes the intrinsic variability in populations: it characterizes the variability in mutation and replication rates in populations of macromolecules; the rate of decay of synchrony in populations of cells; and the degree of iteroparity in populations of individuals. The Malthusian parameter determines ultimate population numbers: under constant environmental conditions, it is the rate of increase when equilibrium conditions are attained. Entropy determines population stability: the gain in the Malthusian parameter due to small fluctuations in the life-cycle variables is determined by entropy. These properties, which are valid for populations of macromolecules, cells and individuals, show that the Malthusian parameter and entropy act as complimentary fitness indices in understanding evolutionary change in populations.     

The units of currency for plant water status

Abstract. The appropriate measure for plant water status is of fundamental importance in plant water relations. One possible approach is to consider a hypothetical sensor for water status at the cell level. It is argued that such a sensor must respond to an intrinsic variable describing the quantity of water present in the cell. Uncertainty about the variable being sensed has affected our interpretion of plant responses to drought. In general, a more rigorous analysis of the effects of tissue hydraulic parameters and further understanding of the pertinent variable for water status are needed to assess such questions. For example, the significance of changes in the elasticity of cell walls in response to drought depends on the measure for water status. A simple model for water transport suggests that a decrease in the elastic modulus may act to maintain turgor for succulent plant species, but it is not clear how increases in the elastic modulus enhance water uptake for non-succulent species.     

The units of DNA replication in the mammalian chromosomes: Evidence for a large size of replication units

The replication of chromosomal DNA in human and Chinese hamster cell populations has been studied by means of the DNA fiber autoradiography. It was found that the rate of DNA replication for one fork in human cells varies from 0.2 to 0.9 m/min, the average being 0.6 m/min. In the Chinese hamster cells the rate of DNA replication is greater, varying from 0.3 to 1.2 m/min, the average being 0.8 m/min. There are no clusters containing a great number of replication units in human and Chinese hamster cells. Sequences consisting of two or three replicons which belong to single DNA molecule have been observed, but their frequency was relatively low. The distances between the initiation points in such sequences of replicons vary from 40 to 280 m, the average value being 130 m. This value represents the minimum size of the replication units which have completed the DNA synthesis within 3 h of the S-period. The DNA synthesis in most replication units fails to be accomplished within the three hours of labelling. The process can be completed only in the fragments of DNA molecules of 40 to 200 m (the average value being 100 m) in human cells, whereas in the Chinese hamster cells the fragments of 40 to 250 m (the average being about 140 m) are completely replicated. Provided that the replication is bidirectional the complete replicons are supposed to contain two such fragments. Consequently, the greater part of replication units in mammalian cells covers the pieces of a few hundred microns in DNA molecules. The relation between replication process at the DNA molecules level and that at the metaphase chromosome level is discussed.     

Thermal resistance units

A proposal is made for standardization of the constant of proportionality used to derive thermal resistances in units of seconds per metre. The choice of a standard value equal to the volumetric specific heat of air at STP would reduce ambiguity in current publications and offer a rational conversion for the clo unit of insulation, as 1 clo = 200 sm^?1 (2 s cm^?1), which gives an alternative easily visualised reference for clothing insulation.     

The 5S DNA units ofAcacia species (Mimosaceae)

The DNA sequence structure of 5S DNA units inAcacia species, including representatives from the three subgenera ofAcacia, have been determined. The data was interpreted to suggest that at least three lineages of 5S DNA sequences exist inAcacia and the proposal was made that the lineages be named5S Dna-1, 5S Dna-2, and5S Dna-3. The5S Dna-1 lineage was represented by units fromA. boliviana andA. bidwilli, the5S Dna-2 lineage by units fromA. melanoxylon, A. pycnantha, A. ulicifolia, A. boliviana, A. bidwillii, andA. albida, and the5S Dna-3 lineage by units fromA. bidwillii, A. boliviana, andA. senegal. Based on this interpretation of the sequence data, the Australian species of subg.Phyllodineae grouped together as a cluster, quite separate from the subgeneraAculeiferum andAcacia. As expected from the analyses of morphological characters, the 5S DNA units fromAcacia albida (syn.Faidherbia albida) were quite separate from the otherAcacia spp.     

The microbial ecology of pilosebaceous units isolated from human skin

A method allowing isolation and microbiological analysis of individual pilosebaceous units (follicles) was used to study biopsies of back skin obtained from volunteer acne vulgaris patients. The main microbial groups isolated were members of the genera Propionibacterium, Staphylococcus and Pityrosporum. The incidence (and mean density) of these organisms in 140 normal follicles was 12% (2.6 X 10(5) per follicle), 4% (5.5 X 10(3) per follicle) and 13% (10(2) per follicle) respectively. Colonized follicles were not distributed evenly amongst the subjects studied. The results are analysed and discussed from an ecological standpoint.     

Accounting units in DNA

Chargaff's first parity rule (%A=%T and %G=%C) is explained by the Watson-Crick model for duplex DNA in which complementary base pairs form individual accounting units. Chargaff's second parity rule is that the first rule also applies to single strands of DNA. The limits of accounting units in single strands were examined by moving windows of various sizes along sequences and counting the relative proportions of A and T (the W bases), and of C and G (the S bases). Shuffled sequences account, on average, over shorter regions than the corresponding natural sequence. For an E. coli segment, S base accounting is, on average, contained within a region of 10 kb, whereas W base accounting requires regions in excess of 100 kb. Accounting requires the entire genome (190 kb) in the case of Vaccinia virus, which has an overall "Chargaff difference" of only 0.086% (i.e. only one in 1162 bases does not have a potential pairing partner in the same strand). Among the chromosomes of Saccharomyces cerevisiae, the total Chargaff differences for the W bases and for the S bases are usually correlated. In general, Chargaff differences for a natural sequence and its shuffled counterpart diverge maximally when 1 kb sequence windows are employed. This should be the optimum window size for examining correlations between Chargaff differences and sequence features which have arisen through natural selection. We propose that Chargaff's second parity rule reflects the evolution of genome-wide stem-loop potential as part of short- and long-range accounting processes which work together to sustain the integrity of various levels of information in DNA.